Relevant sides are discussing the extension of the talks, and the next round of the talks could probably resume in September according to the preliminary plan, but the location has not been decided yet, said the sources.

After rounds of intensive negotiations and the top diplomats meetings, the talks made some progress on some key issues but real differences on some other key issues remain, which could hardly be resolved before July 20.

The diplomatic sources noted the ongoing talks might end up no later than July 19, and the decision of extension might come at earliest on Friday.

The P5+1 states (the five UN Security Council permanent members plus Germany) are engaging Iran to find a comprehensive solution to address the concern on its disputed nuclear program.

In Washington, U.S. President Barack Obama consulted with Secretary of State John Kerry about the extension. He told reporters after the meeting that Tehran had met its commitments under the interim deal but that big gaps remained.

"There are still significant gaps between the international community and Iran and we have more work to do," Obama said in a statement from the White House.

Kerry said earlier in Vienna on Tuesday that more works still need to be done to reach a comprehensive agreement.

Diplomats and experts also believe that all sides need more time to bridge the remaining gaps based on the progress made.

Under the interim deal agreed in Geneva last November, Iran agreed to suspend some sensitive nuclear activities in exchange of limited sanction relief in a duration of six months to buy time for the diplomatic effort to end the deadlock.